Eldon Ng

Transillumination hyperspectral imaging for histopathological examination of excised tissue

Angular domain spectroscopic imaging (ADSI) is a novel technique for the detection and characterization of optical contrast in turbid media based on spectral characteristics. The imaging system employs a silicon micromachined angular filter array to reject scattered light traversing a specimen and an imaging spectrometer to capture and discriminate the largely remaining quasiballistic light based on spatial position and wavelength. The imaging modality results in hyperspectral shadowgrams containing two-dimensional (2D) spatial maps of spectral information. An ADSI system was constructed and its performance was evaluated in the near-infrared region on tissue-mimicking phantoms. Image-based spectral correlation analysis using transmission spectra and first order derivatives revealed that embedded optical targets could be resolved. The hyperspectral images obtained with ADSI were observed to depend on target concentration, target depth, and scattering level of the background medium. A similar analysis on a muscle and tumor sample dissected from a mouse resulted in spatially dependent optical transmission spectra that were distinct, which suggested that ADSI may find utility in classifying tissues in biomedical applications.

Angular domain transillumination imaging optimization with an ultrafast gated camera

By employing high-aspect-ratio parallel microchannels as an angular filter, quasiballistic photons sensitive to internal structures in a turbid medium can be captured. Scattered photons exiting the turbid medium typically exhibit trajectories with random angles compared to the initial trajectory and are mostly rejected by the filter. However, angular filter arrays cannot differentiate between quasiballistic photons (early arriving) and photons that happen to attain a scattered trajectory that is within the acceptance angle (late arriving). Therefore, we have two objectives: (1) to experimentally characterize the angular distribution and proportion of minimally deviated quasiballistic photons and multiply scattered photons in a turbid medium and (2) to combine time and angular gating principles so that early and late arriving photons can be distinguished. From the angular distribution data, the angular filter with angular acceptance about 0.4 deg yields the highest image contrast for transillumination images. The use of angular domain imaging(ADI) with time-gating enables visualization of submillimeter absorbing objects with approximately seven times higher image contrast compared to ADI in a turbid medium with a scattering level of six times the reduced mean free path.

Analysis of a photoacoustic imaging system by singular value decomposition

Photoacoustic imaging is a hybrid imaging modality capable of producing contrast similar to optical imaging techniques but with increased penetration depth and resolution in turbid media by encoding the information as acoustic waves. In general, it is important to characterize system performance by parameters such as sensitivity, resolution, and contrast. However, system characterization can extend beyond these metrics by implementing advanced analysis via singular value decomposition. A method was developed to experimentally measure a matrix that represented the imaging operator for the system. Analysis of the imaging operator was done via singular value decomposition so that the capability of the system to reconstruct objects and the inherent system sensitivity to those objects could be understood. The results provided by singular value decomposition were compared to simulations performed on an ideal system with matching transducer arrangement and defined object space.

Angular domain fluorescent lifetime imaging in turbid media

We describe a novel florescent lifetime imaging methodology applicable to fluorophores embedded in turbid media. The method exploits the collimation detection capabilities of an angular filter device to extract photons emitted by a fluorophore embedded at depth within the medium. A laser source is used to excite the fluorophore within the medium. Photons emitted by the fluorophore that are not scattered to a high degree pass through the angular filter array and are detected by the intensified CCD camera (200 ps minimum gate width). Scattered photons are rejected by the filter and do not pass through to the camera. We fabricated angular filter arrays using silicon bulk micromachining and found that an array of 80 μm square aperture micro-tunnels, 1.5 cm in length accepted photons with trajectories within 0.4° of the axes of the micro-tunnels. The small acceptance angle rejected most of the scattered light exiting the turbid medium.

Three-dimensional angular domain optical projection tomography

Angular Domain Imaging (ADI) has been previously demonstrated to generate projection images of attenuating targets embedded within a turbid medium. The imaging system employs a silicon micro-tunnel array positioned between the sample and the detection system to reject scattered photons that have deviated from the initial propagation direction and to select for ballistic and quasi-ballistic photons that have retained their forward trajectory. Two dimensional tomographic images can be reconstructed from ADI projections collected at a multitude of angles. The objective of this work was to extend the system to three dimensions by collecting several tomographic images and stacking the reconstructed slices to generate a three dimensional volume representative of the imaging target. A diode laser (808nm, CW) with a beam expander was used to illuminate the sample cuvette. An Angular Filter Array (AFA) of 80 μm × 80 μm square-shaped tunnels 2 cm in length was used to select for image forming quasi-ballistic photons. Images were detected with a linear CCD. Our approach was to use a SCARA robot to rotate and translate the sample to collect sufficient projections to reconstruct a three dimensional volume. A custom designed 3D target consisting of 4 truncated cones was imaged and reconstructed with filtered backprojection and iterative methods. A 0.5 mm graphite rod was used to collect the forward model, while a truncated pseudoinverse was used to approximate the backward model for the iterative algorithm.

Angular domain spectroscopic imaging of turbid media using silicon micromachined microchannel arrays

We experimentally characterized a novel Angular Domain Spectroscopic Imaging (ADSI) technique for the detection and characterization of optical contrast abnormalities in turbid media. The new imaging system employs silicon micromachined angular filtering methodology, which has high angular selectivity for photons exiting the turbid medium. The angular filter method offers efficient scattered light suppression at moderate levels of scattering (i.e. up to 6 reduced mean free paths). An ADSI system was constructed from a broadband light source, an Angular Filter Array (AFA), and an imaging spectrometer. The free-space collimated broadband light source was used to trans-illuminate a turbid sample over a wide range of wavelengths in the near infrared region of the spectrum. The imaging spectrometer decomposed the output of the AFA into hyperspectral images representative of spatial location and wavelength. It collected and angularly filtered a line image from the object onto the CCD camera with the spatial information displayed along one axis and wavelength information along the other. The ADSI system performance was evaluated on tissue-mimicking phantoms as well as fresh chicken breast tissue. Collected images with the ADSI displayed differences in image contrast between different tissue types.

Angle-resolved spectroscopy using a radial angular filter array

This paper presents a novel optical filter called the Radial Angular Filter Array (RAFA) for real-time measurement of the angular and spectral distribution of diffuse light exiting a turbid medium. The RAFA consists of a radiallydistributed series of 48 micro-channels micro-machined into a silicon substrate. To test the device, we constructed an angle-resolved spectroscopy system by integrating a wideband light source, the RAFA, and an imaging spectrometer. The collimated broadband light source was configured to trans-illuminate a turbid sample over a wide range of wavelengths in the near infrared spectral region. The RAFA was used to collect the angular distribution of light exiting the turbid sample. The imaging spectrometer decomposed the output of the RAFA into hyperspectral images representative of scatter angle and wavelength. By scanning the RAFA and imaging spectrometer over the sample, the intensity of the scattered light was acquired as a function of location on the sample surface, wavelength, and angle relative to the surface normal. With angle resolved spectroscopy it will be possible to characterize the optical properties of turbid samples in great detail.

Contrast and resolution analysis of angular domain imaging for iterative optical projection tomography reconstruction

Angular domain imaging (ADI) generates a projection image of an attenuating target within a turbid medium by employing a silicon micro-tunnel array to reject photons that have deviated from the initial propagation direction. In this imaging method, image contrast and resolution are position dependent. The objective of this work was to first characterize the contrast and resolution of the ADI system at a multitude of locations within the imaging plane. The second objective was to compare the reconstructions of different targets using filtered back projection and iterative reconstruction algorithms. The ADI system consisted of a diode laser laser (808nm, CW, ThorLabs) with a beam expander for illumination of the sample cuvette. At the opposite side of the cuvette, an Angular Filter Array (AFA) of 80 μm x 80 μm square-shaped tunnels 1 cm in length was used to reject the transmitted scattered light. Image-forming light exiting the AFA was detected by a linear CCD (16-bit, Mightex). Our approach was to translate two point attenuators (0.5 mm graphite rod, 0.368 mm drill bit) submerged in a 0.6% IntralipidTM dilution using a SCARA robot (Epson E2S351S) to cover a 37x37 and 45x45 matrix of grid points in the imaging plane within the 1 cm path length sample cuvette. At each grid point, a one-dimensional point-spread distribution was collected and system contrast and resolution were measured. Then, the robot was used to rotate the target to collect projection images at several projection angles of various objects, and reconstructed with a filtered back projection and an iterative reconstruction algorithm.

Effect of time-gating and polarization-discrimination of propagating light in turbid media during Angular Domain Imaging (ADI)

Angular Domain Imaging (ADI) employs an angular filter array to accept photons within a small acceptance angle along the axis of an aligned laser light source and preferentially reject scattered light. Simulations show that the accepted photons travel the shortest paths between source and detector and are therefore the earliest to arrive. We fabricated angular filter arrays using silicon bulk micromachining and found that an array of 60 μm square shape microtunnels 1 cm in length accepted photons within 0.48 degree of axis of the micro-tunnels. This small acceptance angle rejected most of the scattered light and sub-millimeter resolution targets could be resolved in a few centimeters of turbid medium with at least six times reduced mean free path. ADI through media with higher scattering coefficients was not achievable due to unwanted acceptance of late arriving scattered photons. To reject the late arriving photons, we added time-domain filtration and linear polarization to ADI. The implementation of a time-gated camera, a 780 nm femtosecond pulsed laser, and linear polarization to our ADI system resulted in improved image contrast. The use of ADI with time-gating (gate width 250 ps) and linear polarization enabled visualization of sub-millimeter absorbing objects with approximately eight times higher image contrast compared to ADI in a scattering medium equivalent to six times reduced mean free path.

Two-dimensional angular filter array for angular domain imaging with 3D printed angular filters

Angular Domain Imaging (ADI) is a technique that is capable of generating two dimensional shadowgrams of attenuating targets embedded in a scattering medium. In ADI, an angular filter array (AFA) is positioned between the sample and the detector to distinguish between quasi-ballistic photons and scattered photons. An AFA is a series of micro-channels with a high aspect ratio. Previous AFAs from our group were constructed by micro-machining the micro-channels into a silicon wafer, limiting the imaging area to a one dimensional line. Two dimensional images were acquired via scanning. The objective of this work was to extend the AFA design to two dimensions to allow for two dimensional imaging with minimal scanning. The second objective of this work was to perform an initial characterization of the imaging capabilities of the 2D AFA. Our approach was to use rapid 3D prototyping techniques to generate an array of micro-channels. The imaging capabilities were then evaluated by imaging a 0.9 mm graphite rod submerged in a scattering media. Contrast was observed to improve when a second angular filter array was placed in front of the sample to mask the incoming light.